Technics SU-V9- high pitch sound

[MaccAu]

The V9 is back with a high pitch sound to bleed the ears

After some months of bliss, she was working a charm.

Then one morning after a bit of a break, she fired with a high pitch sound with audio still present, much much louder phono. Upon re fire put a loud sound through speakers *bassy loud* flutter i think.

Yesterday i went through the amp, all seemed fine fixes etc, what i can see, any bad solder work i looked over and it was making connection.

? possible faults rca terminals rubbing cabinet?
? new or original parts failure?
? relay packing in?
? possible issue with briding on phono pcb?
? film cap 2uf output for phono?


I think i am going like the amp lol

what your ears on this demo video of the issue and sound

YouTube


YouTube

 

[jaycee]

that sounds like oscillation. could be faulty decoupling capacitors or a faulty opamp. but you'd need an oscilloscope to know for sure

 

[Mark Tillotson]

Presumably this is one channel only - in which case you can differentially troubleshoot by comparing circuit voltages with the other channel, perhaps swapping things over.

If both channels are affected that implies some inter-channel feedback loop or perhaps PSU issue?

 

[MaccAu]

Thanks Jaycee, for the reply and help. I havent any scope. I changed the capacitors due to leakage, all electros except the main 18000uf caps. I left some ceramic caps, and opamps are OPA1642 on the tone and phono pcb, and speaker impedance NE5532 and also for bias opamp on pre

Both channels, fine the day before then randomly happens again

Thanks Mark, yes both channels , phono as seen in video above is much louder

PSU, new caps, diodes replaced blown thyristors, wima in PSU, Wiring to power button different cable 500v and main in

as per photos

 

[Turbowatch2]

Hi!
Today osciloscopes cost less than any professionel repair of a vintage hifi gadget.
If you are interested in electronics and can identify the hot end of a soldering iron, get a 2 chanel unit, that can deal with about 10 volts minimum (which is easy) and a low MHz count (as audio only needs about 500 kHz). Then you need 2 probes with a 1:1 / 1:10 switch. This makes it capable of measure up to 100 volts in the 1:10 setting.
Thats all. Nothing is less problematic to find a fault in a stereo amp if one side is working.
With your oscillation, work back from the output. Adjust the oscilloscope until you can see the signal at the defective side. Now you compare to the other side, until both show the right signal, in this case no frequency.
There is the problem. A schematic makes it even easyer to find what is wrong.

Good luck!

 

[Turbowatch2]

Sorry, didn´t see your post.
Wow, that is some modified device.
Oscillation can be caused by faulty diodes. If you have a temporary problem, use a hot air gun to activate the fault and an ice spray for deactivate it and localize the part or soldering joint.

 

[MaccAu]

Hello Turbowatch
Thank you for the reply and help.

The diodes are from a Jaycar (not sure of the brand) Before that it had Thyristors. It was temporary, now seems common.

I had to repair crack on board also. Yes, new capacitors, and film cap output on phono.

I will have to look for cheap scope or try to isolate issue. Its a bad sound on the video, very high pitch. I have a heatgun, no ice spray but Jaycar has that.

A few fixes on the boards with briding wiring, which i checked, and i thought i fixed it, and ran it for hours, then the next day it came back and since has been like that everytime i start it

 

[jaycee]

OK, so you have replaced the opamps with more modern ones. From looking at the schematic, there is no HF decoupling on the opamps, so this could be the problem. Try soldering 100nF ceramics across the opamp supply pins 4 and 8

Also by using diodes instead of the thyristors, you may have effectively disabled the impedance detect/switch circuitry.

 

[MaccAu]

Thanks for the reply Jaycee

Yes since i slipped the probe and that took out the thyristors plus resistor and transistors and opamps.

I had to replace all the parts NE5532 for voltage comparator and dc servo and OPA1642, which might need some 100nf caps for tone and phono


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Update on V9
moved to table, moved some wires away from PSU

Fired up and tested CD on tuner setting - didnt sound right and had some static

flicked to phono ( no turntable connected) and there was no sound on the left but right channel was popping

 

[MaccAu]

Fingers crossed phono decouping cap removed so tuner connection works and phono is dead silent.

Have to see whats wrong with the right channel for the phono, mustve affected other connections ???